1. Field of the Invention
The present invention relates to an inspection sensitivity evaluation method. More specifically, for example, it relates to an inspection sensitivity evaluation method of an inspection apparatus that inspects a pattern by irradiating laser lights or electron beams so as to acquire an optical image of the pattern to be inspected.
2. Description of Related Art
In recent years, with the advance of high integration and large capacity of a large scale integrated circuit (LSI), the line width (critical dimension) required for circuits of semiconductor elements is becoming progressively narrower. Such semiconductor elements are manufactured by exposing and transferring a pattern onto a wafer to form a circuit by means of a reduced projection exposure apparatus known as a stepper while using an original or “master” pattern (also called a mask or a reticle, and hereinafter generically referred to as a mask) with a circuit pattern formed thereon. Then, in fabricating a mask used for transferring such a fine circuit pattern onto a wafer, a pattern writing apparatus capable of writing or “drawing” fine circuit patterns by using electron beams needs to be employed. Pattern circuits may be written directly on the wafer by the pattern writing apparatus. Also, a laser beam writing apparatus that uses laser beams in place of electron beams for writing a pattern is under development.
Since LSI manufacturing requires a tremendous amount of manufacturing cost, it is crucial to improve its yield. However, as typified by a 1-gigabit DRAM (Dynamic Random Access Memory), the scale of a pattern configuring an LSI has been changing from an order of submicrons to an order of nanometers. One of major factors that decrease the yield of the LSI manufacturing is a pattern defect of a mask used when exposing and transferring a fine pattern onto a semiconductor wafer by the photolithography technology. In recent years, with miniaturization of dimensions of an LSI pattern formed on a semiconductor wafer, dimensions to be detected as a pattern defect have become extremely small. Thus, a pattern inspection apparatus which inspects a defect of a transfer mask used in manufacturing LSI needs to be highly precise.
As an inspection method, there is known a method of comparing an optical image of a pattern, formed on a target object or “sample”, such as a lithography mask, imaged at a predetermined magnification by using a magnifying optical system with design data or an optical image obtained by imaging the same pattern on the target object. For example, the following is known as pattern inspection methods: the “die-to-die inspection” method that compares data of optical images of identical patterns at different positions on the same mask; and the “die-to-database inspection” method that inputs, into the inspection apparatus, writing data (design pattern data) which is generated by converting pattern-designed CAD data to a writing apparatus specific format for input when writing a pattern on the mask, generates design image data (reference image) based on the input writing data, and compares the generated design image data with an optical image (serving as measurement data) obtained by imaging the pattern. According to the inspection method for use in such an inspection apparatus, a target object is placed on the stage so that a light flux may scan the object by the movement of the stage in order to perform an inspection. Specifically, the target object is irradiated with a light flux from the light source and the illumination optical system. Light transmitted through the target object or reflected therefrom is focused on a sensor through the optical system. An image captured by the sensor is transmitted as measurement data to the comparison circuit. In the comparison circuit, after performing position alignment of images, measurement data and reference data are compared with each other in accordance with an appropriate algorithm. If there is no matching between the compared data, it is determined that a pattern defect is present.
In pattern inspection, it is also required to measure a critical dimension (CD) deviation of a pattern and a positional deviation of a pattern in addition to inspecting a pattern defect (shape defect). Conventionally, a dedicated measuring device is used for measuring a critical dimension (CD) deviation of a pattern or a positional deviation of a pattern. If these deviations can be simultaneously measured when performing a pattern defect inspection, it will be a great advantage in cost wise and inspection time wise. Therefore, the inspection apparatus is increasingly requested to have such a measurement function. With regard to measurement of a CD deviation, there is proposed an inspection method in which a pattern line width (critical dimension) in an image obtained for each preset region is measured, a difference from design data is calculated, and an average of all the CD differences in a region is compared with a threshold value, so that a line width abnormal region is found as a CD error (measurement defect) (refer to, e.g., Japanese Patent No. 3824542).
For realizing the measurement of a CD deviation and a positional deviation by an inspection apparatus, it is necessary to measure deviation of a pattern formed on the real mask with respect to mask design data. Meanwhile, precision of a CD deviation amount or a positional deviation amount measured by the inspection apparatus is verified by comparison with data measured by a dedicated measuring device. With the current trend of miniaturization of patterns, precision (level) of several nanometers is required as the precision of CD deviation or positional deviation. Thus, similarly, the level of several nanometers is also required for inspection sensitivity (measurement sensitivity) of the inspection apparatus. Therefore, in order to evaluate the inspection sensitivity of the inspection apparatus, it is needed to fabricate a real mask for evaluation where CDs and positions are shifted, for example, by about 1/10 of the specification value, such as by 0.1 to 0.2 nm. However, there is a problem that to practically fabricate a real mask on which patterns are formed with the CDs and positions shifted at such precision is difficult.
That is, there exists a problem of difficulty in practically fabricating a real mask where patterns are formed in a manner such that CDs and positions are shifted with the precision required for inspection sensitivity of the inspection apparatus in connection with recent miniaturization trend of patterns. Accordingly, for realizing measurement of a CD deviation and a positional deviation by an inspection apparatus, there is a problem that to evaluate inspection sensitivity of the apparatus is difficult.